Processing for corrosion resistance in aluminum base alloys containing zinc, magnesium, iron, and cadmium, tin or lead

ABSTRACT

A method for increasing the corrosion resistance of aluminum base alloys is disclosed. The alloys contain up to 2.0% by weight zinc, up to 5.0% by weight magnesium, less than 0.1% by weight iron, elements selected from the group consisting of cadmium, tin and lead, or any combination thereof, in amounts ranging from 0.05 to 0.3% by weight for each of cadmium and tin and from 0.01 to 0.15% by weight for lead, balance aluminum. These aluminum base alloys are particularly useful as anode material for primary electric cells. The aluminum alloys reduce the incidence of hydrogen gas evolution within the cells. The aluminum base alloys may also be utilized as anode material for uses other than primary cells.

This is a Division of application Ser. No. 707,186, filed July 21, 1976.

BACKGROUND OF THE INVENTION

The present invention relates to a method for increasing the corrosionresistance of aluminum base alloys, which are particularly useful inprimary cells of the "dry" type. The alloys are utilized as the anodematerial in said primary cells, said anode material also serving as thecontainer for the cell. The alloys may also be utilized as anodematerial in applications which require resistance to corrosion, such aswater heaters.

Zinc is extensively employed as anode material in the construction ofdry primary cells, for example, in common flashlight batteries. Numerousproposals have been made to substitute aluminum or aluminum alloys forzinc as the anode material in dry cells in order to utilize the numerousadvantageous properties of the aluminum or aluminum alloys. Aluminum andaluminum alloys are generally less expensive than zinc. As zinc becomesmore and more scarce, this price differential will increase. Aluminumand aluminum alloys also enjoy a greater ease of fabrication to thingages and particularly to formed dry cell battery cases.

Dry cell batteries containing aluminum, aluminum-zinc alloys or otheraluminum base alloys as the anode material have, however, suffered fromnumerous significant disadvantages. Such cells generally require theplacement of a semi-permeable membrane within the battery container toprevent the evolution of large volumes of hydrogen gas resulting fromthe aluminum-electrolyte reaction which takes place within the dry cellbattery. Dry cell battery containers which have been manufactured fromstandard commercial aluminum alloys such as Aluminum Association Alloy1100 are subject to unacceptable hydrogen gas evolution when used. Suchhydrogen evolution causes the battery containers to either swell or, inextreme circumstances, to burst. Either condition is not acceptable tothe ultimate consumer, since a swollen battery usually cannot be removedfrom a device into which it has originally been inserted and a batterywhich has burst may subject the consumer to dangerous corrosivechemicals.

A MENTIONED ABOVE, THIS EVOLUTION OF HYDROGEN GAS HAS PRESENTED TOPROBLEM WHICH THE PRIOR ART HAS ATTEMPTED TO SOLVE THROUGH THE USE OF ASEMI-PERMEABLE MEMBRANE WITHIN THE BATTERY CASE WHICH RETAINS THEELECTROLYTE MATERIAL AWAY FROM THE ALUMINUM OF THE ANODE CASING. Such asolution has not been successful with the typical commercial aluminumalloys. Composite alloys which have been utilized to overcome the gasevolution disadvantage have not been entirely satisfactory and are alsoquite expensive.

The evolution of hydrogen gas from the former aluminum alloys has beenthe result of localized attack of said alloys. This attack, generallycaused by the electrolyte utilized in the cell, is a classic example ofaluminum alloy corrosion. Therefore, an aluminum alloy which can reducethe residence of hydrogen gas evolution can also be more resistant tocorrosion than aluminum alloys formerly used. This resistance tocorrosion can be useful in other applications such as anodes for waterheaters and other operations which require resistance to corrosion.

It is, therefore, a primary object of the present invention to provide amethod of increasing the corrosion resistance of aluminum base alloys.

It is a further object of the present invention to provide a method asabove which enables improved aluminum base alloys to be useful as theanode container material for primary electric cells of the dry type.

It is a further object of the present invention to provide a method asabove which enables improved aluminum base alloys to reduce theincidence of hydrogen gas evolution within primary cells as describedabove.

Further objects and advantages of the present invention will appear froma consideration of the following discussion.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has now been found that theforegoing objects and advantages may be readily accomplished by a methodutilizing an aluminum base alloy containing a maximum of 2.0% by weightof zinc, a maximum of 5.0% by weight of magnesium, less than 0.1% byweight iron, concentrations of cadmium, tin and lead in amounts rangingfrom 0.05 to 0.3% by weight of each of cadmium and tin and from 0.01 to0.15% by weight for lead, balance aluminum.

The improved aluminum base alloy utilized in the present inventioncontains preferably less than 0.1% by weight of iron. The iron formscathodic intermetallic compounds with aluminum which tend to segregateto the grain boundaries of the alloy matrix, at which location it isbelieved that hydrogen evolution initiates. The iron content of thealloys should, therefore, be as low as possible. Aluminum base alloyscontaining very much less than 0.1% by weight of iron are, however,impractical from a commercial cost standpoint. On this basis, a limit of0.1% by weight of iron in the aluminum base alloy of the presentinvention is considered a good compromise between the cost of the alloymaterial and the necessary performance characteristics of the formedmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view, in section, illustrating a dry cellstructure embodying the alloy utilized in the present invention.

FIG. 2 is a broken sectional view illustrating the sealing and ventingmechanism in the dry cell structure of FIG. 1.

FIG. 3 is a diagrammatic view, partly in section, illustrating theapparatus utilized for measuring hydrogen evolution of the aluminum basealloys utilized in the present invention and comparative alloys duringgalvanostatic polarization.

DETAILED DESCRIPTION

The aluminum base alloy used in the present invention contains a maximumof 2.0% by weight zinc, a maximum of 5.0% by weight magnesium, less than0.1% by weight iron, and elements selected from the group consisting ofcadmium, tin and lead, or any combination thereof, in amounts rangingfrom 0.05 to 0.03% by weight of each of cadmium and tin and from 0.01 to0.15% by weight for lead, balance aluminum. Preferably, the alloycontains from 0.5 to 1.0% by weight zinc, a maximum of 1.0% by weightmagnesium, a maximum of 0.05% by weight of iron, and an element selectedfrom the group consisting of cadmium, tin and lead, or any combinationthereof, in amounts ranging from 0.05 to 0.15% by weight for each ofcadmium and tin and from 0.02 to 0.05% by weight lead, balance aluminum.The optimum zinc concentration in the alloy is approximately 1.0% byweight. The optimum magnesium concentration in the alloy isapproximately 0.5% by weight. The optimum tin and cadmium concentrationsin the alloy are approximately 0.1% by weight of each element. Theoptimum lead concentration in the alloy is approximately 0.1% by weight.

High purity aluminum may be employed in the alloys used in the presentinvention. It should be noted, however, that high purity aluminum ismuch less economical than commercially available aluminum which wouldgenerally fulfill the same requirements as would high purity aluminum.Therefore, it is preferred in the instant invention to utilize lowerpurity aluminum as the base aluminum of the alloy system. This aluminum,as commercially available, generally contains from 0.001 to 0.1% byweight silicon, and from 0.001 to 0.1% by weight iron. As noted above,it is critical that the percentage of iron in the alloy system used inthe present invention remain less than 0.1%, preferably less than 0.05%by weight of the alloy. The percentage of silicon in the alloy systemused in the present invention should remain less than 0.1%, preferablyless than 0.5% by weight of the alloy. The lower purity aluminum may besubstituted for high purity aluminum without detriment to theelectrochemical characteristics of the resulting alloy.

It should be understood that the alloys used in the present inventionmay contain, in addition to the elements described above, otherimpurities normally found in commercially available aluminum. Theseother materials may also be added to the alloy used in the presentinvention to achieve particularly desirable results. These materialsshould, however, be limited in amounts which may affect the anodicefficiency of the alloy by forming second phase particulate cathodes andthus promoting localized corrosion of the anode.

Any suitable cathode may be employed in the dry cell utilizing the alloysystem of the present invention. For example, the conventional carbon orgraphite cathodes may be utilized. These cathodes are usually used witha conventional cathodic depolarizer such as manganese dioxide.

The various electrolytes suggested in the art for use in dry cells maybe conveniently used in the primary cell utilizing the alloy system usedin the present invention. Aluminum chloride is the preferred electrolytefor use in primary cells formed from the alloy system utilized in thepresent invention because of the common aluminum ion. The electrolyteswhich have been used for zinc primary cells, ammonium chloride and zincchloride, would be more corrosive in the aluminum cells because of theabsence of the common ion.

Referring to FIG. 1, which illustrates an embodiment utilizing the alloysystem utilized in the present invention, outer case 1, which forms thecovering material for the dry cell, is formed from cardboard or similarmaterial. Directly inside case 1, but spaced therefrom near the top ofthe cell, is shell 2 formed from the aluminum base alloy utilized in thepresent invention. Contacting said shell and held in place by said outercase is a metallic formed disc 3, which is in electrical contact withsaid alloy shell. The base of said shell is covered with a plasticinsulating disc 4 and the disc is covered with a shallow polymer coatedKraft paper cup 5. The sidewalls of shell 2 are covered on the sideopposite from said case 1 with an ion permeable fibrous separator sheet6. The polymer coated Kraft paper cup 5 is pressed against the bottom ofsaid separator sheet 6 to prevent migration of cathode mix particles tothe aluminum alloy shell 2 at the junction of the plastic insulatingdisc 4 and the separator sheet 6. The separator sheet covered alloyshell is filled with a conventional cathodic depolarizer 7 such as amixture of manganese dioxide and acetylene carbon black. A liquidelectrolyte is generally added to the depolarizer mixture to activatethe dry cell. A carbon rod 8 is inserted in the center of the mixtureand forms the cathode current collector for the cell. A plastic disc 9is placed over the mixture and extends outwardly to the case 1 fromcarbon rod 8. The dry cell is generally capped with a metallic disc 10formed so as to cover the cathode collector rod 8 and seal the dry cellat the perimeter of the shell 2 and case 1.

FIG. 2 is an enlarged view of the sealing and spacing arrangement in thedry cell illustrated in FIG. 1. A space is provided between the endportion of shell 2 and the end portion of case 1 to collect any hydrogengas which may evolve from the interaction of the electrolyte paste 7with shell 2 through the porous separator sheet 6.

FIG. 3 illustrates the test apparatus utilized to measure the relativehydrogen evolution rates of the aluminum base alloys utilized in thepresent invention and comparative alloys in a dry battery simulationwhile in use with electric current being drawn and with the electriccurrent shut off. The apparatus was also used to measure the electrodepotential of the aluminum alloy specimen which was a measure of the drycell voltage which would be available if the alloys were used as abattery can. The specimen 1' consisted of a square piece of the sheetmaterial consisting of the experimental aluminum alloy. An insulatedcopper lead wire 2' was connected from the specimen 1' to the counterelectrode terminal 3' of a potentiostat 4'. The insulated copper leadwire 2' and the edge of the specimen 1' were masked with electricallyinsulating lacquer, thus leaving exactly ten square centimeters ofexposed alloy material.

The specimen 1' was gripped in a groove formed in a rubber stopper 5'placed at the bottom of a glass petri dish 6'. A platinum mesh cathode7' was inserted in the petri dish so that the mesh surrounded thespecimen 1'. This cathode was connected to the reference terminal 8' ofthe potentiostat 4'. The petri dish was filled with a 12.5% aluminumchloride mixture 9' so that the specimen 1' was submerged 1.5 inchesbelow the mixture surface. Saturated Calomel (Hg₂ Cl₂) 10' was placedinside a glass separatory funnel 11 which in turn was connected to acapillary tube 12 positioned within one-sixteenth of an inch from thespecimen surface. A glass stopper 13, placed between the separatoryfunnel 11 and the capillary tube 12, was lubricated by means of a slurryof agar mixed with sodium chloride and bentonite which was capable ofconducting electricity. A 17 ohm (Ω) resistance was placed between thereference 8' and working 14 terminals of the potentiostat 4'. A gasburette 15 with a glass funnel fused to its bottom portion was placedover the specimen. Air was pumped out of the gas burette until theliquid 9' from the petri dish filled the entire burette. At this point,the stopper 16 on the top portion of the burette was closed. The Calomelshielded electrode lead wire 17 was connected to the positive side of anelectrometer equipped with a coaxial lead wire. The grounded shield ofthis coaxial lead wire was connected to the lead wire 2' betweenspecimen 1' and counter terminal 3'.

The alloys utilized in the present invention may be cast according toany convenient procedure, including DC and Durville casting. The castalloys are homogenized for 4 to 30 hours at 1050°-1125° F (565.6°-600°C), preferably for 24 hours at 1100° F (593.3° C). The homogenizedalloys are then rapidly cooled, preferably with water. The cooled alloysare scalped down to a reasonable working size and are subsequently hotworked at 850° to 1000° F (454.4 to 537.8° C) with a reduction ofapproximately 20% during each pass through the hot working station downto approximately 0.2 inch. The hot worked alloys are then cold workedwith a reduction of approximately 20% during each pass through the coldworking station down to approximately 0.1 inch.

The alloys are then annealed for 5 to 30 minutes at 750° to 1050° F(398.9° to 565.6° C), preferably 15 minutes at 950° F (510° C). Theannealed alloys are then finally cold worked down to the desired workingthickness, preferably approximately 0.02 inch.

After being cold worked down to the final desired working thickness, thealloys are subjected to a final anneal. The alloys are finally annealedfor 1 to 25 minutes at 100° to 800° F (37.8° to 426.7° C).

It should be noted that both the hot working and cold working steps maybe repeated a number of times to reduce the alloys to desired workingthicknesses in each step.

The improvements presented by the method of the present invention willbecome more apparent from a consideration of the following examples.

EXAMPLE I

An alloy of 1.0% by weight zinc, 0.5% by weight magnesium and 0.1% byweight cadmium and balance aluminum was cast. The cast alloy wassubjected to a 24 hour homogenization treatment at 1100° F followed bywater cooling. The cast ingots were scalped to 2.5 inches equally onboth sides of the ingots. The scalped ingots were hot rolled at 900° Fin 20% passes down to 0.2 inch. The hot rolled material was then coldrolled in 20% passes down to 0.1 inch. The cold rolled material wasannealed for 15 minutes at a furnace temperature of 950° F followed bycold rolling down to 0.02 inch. The worked material was divided intothree portions for testing. The first portion was finally annealed for15 minutes at 150° F. The second portion was finally annealed for 15minutes at 600° F. The third portion was finally annealed for 5 minutesat 700° F. Data representing the hydrogen gas evolved from samples ofeach annealed portion were obtained from operation of the apparatusdescribed in FIG. 3. The results are presented in Table I.

                  TABLE I                                                         ______________________________________                                        Hydrogen Evolution from Different Anneals of                                   Al-0.5% Zn-1.0% Mg-0.1% Cd                                                                Hydrogen Evolution,                                                           cc per 10 cm.sup.2                                               Casting                                                                             Partial      Three Hours With                                                                            One Hour With                                Portion                                                                             Anneal       Current On    Current Off                                  ______________________________________                                        First 150° F × 15 min.                                                              15.4          0.3                                          Second                                                                              600° F × 15 min.                                                              16.3          0.3                                          Third 700° F ×  5 min.                                                              14.2          1.2                                          ______________________________________                                    

The data indicate that the 1.0% by weight zinc, 0.5% by weight magnesiumand 0.1% by weight cadmium alloy can be given a partial anneal at 150° Ffor 15 minutes without suffering an increase of hydrogen evolution withcurrent on compared to the material annealed at 600° F for 15 minutes.There is, however, an increase in hydrogen evolution in the current onload relative to the material annealed for 5 minutes at 700° F.

EXAMPLE II

A series of aluminum base alloys, whose composition is shown in TableII, were cast as Durville ingots with an aluminum base containingbetween 0.03 and 0.06% by weight iron and between 0.03 and 0.06% byweight silicon. The ingots were homogenized at 1100° F (593.3° C) for 24hours and then water quenched. The ingots were scalped on both sides toa thickness of 1.5 inches. The slabs were then hot rolled in 20%reduction passes to 0.10 inch gage and subsequently cold rolled to afinal gage of 0.018 inch.

                  TABLE II                                                        ______________________________________                                        Nominal Compositions of Experimental Alloys                                   (Balance Essentially Aluminum)                                                Alloy % Cadmium % Tin   % Lead % Zinc                                                                              % Magnesium                              ______________________________________                                        A     --        --      --     --    --                                       B     0.2       --      --     --    --                                       C     0.2       --      --     1.0   0.5                                      D     0.1       --      --     1.0   0.5                                      E     --        --      --     1.0   0.5                                      F     --        --      --     1.0   --                                       G     0.1       --      --     0.5   1.0                                      H     --        --      --     0.5   1.0                                      I     0.1       --      --     0.5   2.5                                      J     --        --      --     0.5   2.5                                      K     --        --      --     --    2.5                                      L     0.1       --      --     0.5   5.0                                      M     --        --      --     0.5   5.0                                      N     --        0.2     --     --    --                                       O     --        0.1     --     0.5   1                                        P     --        0.1     --     1     0.5                                      Q     --        0.1     0.03   1     0.5                                      R     --        --      0.03   1     0.5                                      S     --        --      0.03   1     --                                       T     --        --      --     1     --                                       U     --        0.1     --     1.7   2.5                                      V     --        --      --     1.7   2.5                                      W     --        --      0.1    --    --                                       X     --        --      0.2    --    --                                       Y     --        --      0.1    5     --                                       Z     --        --      --     5     --                                       ______________________________________                                    

EXAMPLE III

The sheet materials described in Example II were degreased and placedone at a time in the apparatus illustrated in FIG. 3. The potentiostatand electrometer of the apparatus were turned on and adjusted so thatthe current flow through the galvanic cell consisting of the specimenplate, electrolyte and platinum mesh cathode was exactly 100milliamperes. The hydrogen gas evolved during a 3 hour period with thegalvanic cell on current load and a subsequent one hour period with thecurrent load removed was measured and the results are shown in TableIII.

                  TABLE III                                                       ______________________________________                                        Anode Performance With and Without Current Load                                                    One Subsequent Hour                                      Three Hours With Current On                                                                        With Current Off                                               Hydrogen   Electrode   Hydrogen                                               Evolution, Potential,  Evolution,                                       Alloy cc         volts       cc                                               ______________________________________                                        A     27.7       -0.539      1.9                                              B     18.0       -0.629      1.4                                              C     14.7       -0.667      1.9                                              D     14.2       -0.668      1.2                                              E     16.1       -0.660      1.5                                              F     19.2       -0.676      2.8                                              G     15.9       -0.600      0.6                                              H     13.8       -0.598      0.2                                              I     15.1       -0.614      0.5                                              J     14.7       -0.580      0.5                                              K     19.5       -0.501      1.4                                              L     16.2       -0.607      0.6                                              M     14.5       -0.608      0.2                                              N     19.0       -0.594      6.0                                              O     16.5       -0.576      2.4                                              P     14.4       -0.654      1.9                                              Q     14.6       -0.668      2.6                                              R     16.1       -0.679      1.8                                              S     16.3       -0.655      1.9                                              T     19.2       -0.676      2.8                                              U     23.4       -0.710      4.5                                              V     26.5       -0.698      5.6                                              W     19.3       -0.672      0.7                                              X     21.5       -0.541      1.2                                              Y     39.3       -0.740      8.0                                              Z     29.6       -0.748      6.4                                              ______________________________________                                    

The data indicate that alloys containing 0.1 or 0.2% by weight cadmium,zero or 0.1% by weight tin or zero or 0.03% by weight lead in thealuminum base alloyed with up to 1% by weight zinc or up to 5.0% byweight magnesium or combinations of these when processed according tothe present invention, have lower hydrogen evolution rates than thosealloys outside the working ranges for cadmium, tin and lead. Even thoughsome alloys which fall outside of the ranges used in the instantinvention (J, X) exhibit lower hydrogen evolution values than somealloys which do fall within the instant invention ranges, these outsidealloys generally exhibit a lower electrode potential in magnitude thanthe alloys used in the present invention.

It can be seen from Examples II and III that, even thoughaluminum-zinc-magnesium alloys are known broadly as Aluminum Association7XXX series alloys, the restricted range of zinc found necessary in thealloy system utilized in the primary cell of the present invention toachieve low hydrogen evolution rates is not represented by anycommerical alloy. The data presented in Examples II and III indicatethat the alloy system used in the primary cell of the present inventionexhibits a combination of low hydrogen evolution and greater electrodepotential in magnitude than do other aluminum-zinc-magnesium alloysoutside the invention ranges.

EXAMPLE IV

An alloy containing 1.0% by weight zinc, 0.1% by weight tin, 0.5% byweight magnesium, balance aluminum, was formed into a sacrificial anodefor a hot water heater. The performance of this alloy, whose range ofelements falls within the range presented by the alloy system utilizedin the present invention, was compared to the performance of an anodeformed from Aluminum Association Alloy 8020 (0.06% Si max., 0.10% Femax., 0.16-0.22% Sn, 0.10-0.20% Bi, 0.003-0.01 B, 0.01% Ga min., 0.03%Mg max., 0.004% Ti max., max. of 0.005% for each of Cu, Mg, Cr, Ni andZn). Both anodes were placed in hot water heaters containing water at atemperature held within 110° F ± 50° throughout a two week experiment.The hot water heaters expended approximately 100 gallons of hot waterdaily throughout the two week test. This amount of hot water isconsidered normal for an average family. The weight loss of each anodewas measured as well as the total coulombic output of each anode and anefficiency for each anode was measured according to the coulombic outputfor each loss. The results of each anode are shown in Table IV.

                                      TABLE IV                                    __________________________________________________________________________    Coulombic Output Efficiency Comparison of Aluminum Alloy Anodes in Hot        Water                                                                                             Average                                                                            Current                                                                            Total Coulombic                                 Weight Loss         Current,                                                                           Density                                                                            Output    Output Theoretical                    Anode  gms. lbs.    ma   ma/in..sup.2                                                                       amp-secs. amp-hrs/lb.                                                                          Efficiency                     __________________________________________________________________________                                                   %                              AA 8020                                                                              3.30 7.26 × 10.sup.-3                                                                15.8 0.21 19,112    730    54                             Al-1% Zn-                                                                            0.948                                                                              2.09 × 10.sup.-3                                                                 5.8 0.12  7,039    939    70                             0.1% Sn-                                                                      0.5% Mg                                                                       (Nominal)                                                                     __________________________________________________________________________

As can clearly be seen from Table IV, the anode formed by the method ofthe present invention demonstrates superior efficiency and resistance toweight loss compared to material normally used for this purpose.

EXAMPLE V

The anode material containing elements within the ranges utilized in thepresent invention could not be directly compared to an alloy usedcommercially as anode material which contains 4.5% by weight zinc, 0.14%by weight tin, 0.11% by weight iron, balance aluminum. Therefore, acomparison was made between AA Alloy 8020 and this commerciallyavailable material. Both anodes were subjected to a 48 hour immersion ina 0.1M NaCl solution at 25° C ± 2°. As in Example IV, the weight loss,total coulombic output and theoretical efficiency based upon output perunit of weight loss were measured for each anode. The results are shownin Table V.

                                      TABLE V                                     __________________________________________________________________________    Coulombic Output Efficiency Comparison of Aluminum Alloy Anodes in 0.1M       NaCl Solution                                                                                     Average                                                                            Current                                                                            Total Coulombic                                 Weight Loss         Current,                                                                           Density                                                                            Output    Output Theoretical                    Anode   gms. lbs.   ma   ma/in..sup.2                                                                       amp-secs. amp-hrs/lb.                                                                          Efficiency                     __________________________________________________________________________                                                   %                              AA 8020 0.21 4.5 × 10.sup.-4                                                                8.60 5.60 1,496     924    68                             Al-4.5% Zn-                                                                           0.024                                                                              5.3 × 10.sup.-5                                                                0.62 0.40   104     570    42                             0.14% Sn-                                                                     0.11% Fe                                                                      __________________________________________________________________________

The results shown in Table V allow an indirect comparison to be madebetween Al-1.0% Zn-0.1% Sn-0.5% Mg from Example IV and Al-4.5% Zn-0.14%Sn-0.11% Fe from Example V insofar as theoretical efficiency of each canbe determined. As can clearly be seen from Table V, and a review ofTable IV, the anode within the ranges of the present invention is evenmore superior to the commercially available alloy of Example V than itis to AA Alloy 8020. Therefore, the anodes formed by the method of thepresent invention will provide significant protection in even aggressive(i.e., those containing solid or other corrosive materials) waters thancommercial anodes presently utilized for the same purpose.

It can be seen from all the examples presented herein that the primarycell anode material and the other anode material formed by the method ofthe present invention provides performance which is clearly superior tomaterial presently used for the same purposes.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:
 1. A method of preparing an aluminum base alloywhich is resistant to corrosion comprising the steps of:a. casting analloy consisting essentially of up to 2.0% by weight zinc, up to 5.0% byweight magnesium, less than 0.1% by weight iron, an element selectedfrom the group consisting of cadmium, tin and lead, or any combinationthereof, in amounts ranging from 0.05 to 0.3% by weight for each ofcadmium and tin and from 0.01 to 0.20% by weight for lead, balancealuminum; b. homogenizing said cast alloy for 4 to 30 hours at °1125° F;c. rapidly cooling said homogenized alloy; d. hot working the quenchedalloy at 850° to 1000° F with a reduction of approximately 20% duringeach pass through the hot working station; e. cold working the alloywith a reduction of approximately 20% during each pass through the coldworking station; f. heating the cold worked alloy for 5 to 30 minutes at750° to 1050° F; g. cold working said alloy down to the final desiredworking thickness; and h. heating said cold worked alloy for 1 to 25minutes at 100° to 800° F.
 2. A method according to claim 1 wherein saidhot working is performed in step (d) in at least two passes.
 3. A methodaccording to claim 1 wherein said cold working is performed in steps (e)and (g) in at least two passes.
 4. A method according to claim 1 whereinsaid cooling is with water.
 5. A method according to claim 1 wherein thehot working of step (d) is performed to reduce the thickness of saidalloy down to approximately 0.2 inch.
 6. A method according to claim 1wherein the cold working of step (e) is performed to reduce thethickness of said alloy down to approximately 0.2 inch.
 7. A methodaccording to claim 1 wherein the cold working of step (g) is performedto reduce the thickness of said alloy down to approximately 0.02 inch.8. A method according to claim 1 wherein said alloy additionallycontains from 0.001 to 0.1% by weight silicon.
 9. A method according toclaim 1 wherein said alloy contains up to 1.0% by weight magnesium. 10.A method according to claim 1 wherein said alloy contains from 0.5 to1.0% by weight zinc.
 11. A method according to claim 1 wherein saidalloy contains from 0.001 to 0.1% by weight iron.
 12. A method accordingto claim 1 wherein the cadmium and tin are present in said alloy inamounts ranging from 0.05 to 0.15% by weight for each element.
 13. Amethod according to claim 1 wherein the lead is present in said alloy inan amount ranging from 0.02 to 0.10% by weight.